1. Field of the Invention
This invention relates to an optical deflecting apparatus for generating surface acoustic waves in an optical waveguide and deflecting an optical wave guided through the optical waveguide by diffracting actions of the surface acoustic waves. This invention particularly relates to an optical deflecting apparatus wherein a wide deflection angle range is obtained by deflecting the guided optical wave twice by use of two surface acoustic waves.
2. Description of the Prior Art
As disclosed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-183626, there has heretofore been known an optical deflecting apparatus wherein light is made to enter an optical waveguide formed of a material allowing propagation of a surface acoustic wave therethrough, a surface acoustic wave is generated in a direction intersecting the guided optical wave advancing inside of the optical waveguide to effect Bragg diffraction of the guided optical wave by the surface acoustic wave, and the frequency of the surface acoustic wave is continuously changed to continuously change the angle of diffraction (deflection angle) of the guided optical wave. The optical deflecting apparatus of this type is advantageous in that the apparatus can be fabricated small and light and has high reliability because of the absence of mechanical operating elements as compared with a mechanical type optical deflector such as a galvanometer mirror or a polygon mirror, and an optical deflector using an optical deflecting device such as an electro-optic deflector (EOD) or an acousto-optic deflector (AOD).
However, the aforesaid optical deflecting apparatus has the drawback that the deflection angle cannot be adjusted to be large. Specifically, with the optical deflecting apparatus using the optical waveguide, the optical deflection angle is approximately proportional to the frequency of the surface acoustic wave, and therefore the frequency of the surface acoustic wave must be changed up to a very large value in order to obtain a large deflection angle. Thus it is necessary to change the frequency of the surface acoustic wave over a wide band. Also, in order to satisfy the Bragg condition, it is necessary to control the angle of incidence of the guided optical wave upon the surface acoustic wave by continuously changing (steering) the direction of advance of the surface acoustic wave.
In order to satisfy the aforesaid requirements, as disclosed in, for example, the aforesaid Japanese Unexamined Patent Publication No. 61(1986)-183626, there has heretofore been proposed an optical deflecting apparatus wherein a plurality of interdigital transducers (hereinafter abbreviated as IDT) generating surface acoustic waves, the frequency of which changes continuously in frequency bands different from one another, are disposed so that the directions of generation of the surface acoustic waves are different from one another, and the respective IDTs are operated through switching.
However, the proposed optical deflecting apparatus having the aforesaid configuration has the drawback that the diffraction efficiency decreases around the cross-over frequency of the surface acoustic waves generated by the respective IDTs, and therefore the optical amount of the deflected optical wave fluctuates in accordance with the deflection angle.
Also, with the aforesaid configuration, the IDT which bears the portion of a large deflection angle must ultimately be constituted to be able to generate the surface acoustic wave of a very high frequency. This problem will be described below. The deflection angle .delta. of the guided optical wave caused by the acousto-optic interaction between the surface acoustic wave and the guided optical wave is expressed as .delta.=2.theta. wherein .theta. denotes the angle of incidence of the guided optical wave with respect to the direction of advance of the surface acoustic wave. Also, the formula ##EQU1## applies wherein .lambda. and Ne respectively denote the wavelength and the effective refractive index of the guided optical wave, and .LAMBDA., f and v respectively denote the wavelength, the frequency and the velocity of the surface acoustic wave. Therefore, a deflection angle range .DELTA.(2.theta.) is expressed as EQU .DELTA.(2.theta.)=.DELTA.f..lambda./Ne.v.
For example, in order to obtain a deflection angle range .DELTA.(2.theta.) equal to 10.degree. in the case where .lambda.=0.78 .mu.m, Ne=2.2 and v=3,500 m/s, it is necessary that the frequency range .DELTA.f of the surface acoustic wave, i.e. the frequency band of the high frequency applied to the IDT, be .DELTA.f=1.72 GHz. In the case where said frequency band is of one octave so that adverse effects of the second-order diffracted optical wave component can be avoided, the center frequency f0 is equal to 2.57 GHz and the maximum frequency f2 is equal to 3.43 GHz. The period .LAMBDA. of the IDT that gives said maximum frequency f2 is equal to 1.02 .mu.m, and the line width W of the IDT finger is equal to .LAMBDA./4=0.255 .mu.m.
With current photolithography and electron beam drawing processes which are popular techniques for forming the IDT, the possible line widths are limited respectively to approximately 0.8 .mu.m and approximately 0.5 .mu.m. Therefore, it is not always possible to form an IDT having the very small line width mentioned above. Even if such an IDT having the very small line width mentioned above could be formed in the future, a driver for generating a high frequency of approximately 3.43 GHz cannot always be manufactured or can only be done at a very high cost. Also, it is not always possible to apply a high voltage to such an IDT. Further, in the case where the frequency of the surface acoustic wave is increased as mentioned above, the wavelength of the surface acoustic wave naturally becomes short, and therefore the surface acoustic wave is readily absorbed by the optical waveguide and the diffraction efficiency deteriorates.
On the other hand, an optical deflecting apparatus wherein, instead of operating a plurality of IDTs through switching as mentioned above, a single IDT is constituted as a curved-finger chirped IDT in which the transducer finger line width is changed continuously and the respective transducer fingers are in a circular arc shape, and the frequency of the surface acoustic wave and the direction of advance thereof are changed continuously over a wide range by the single IDT is disclosed in IEEE Transactions on Circuits and Systems, Vol. CAS-26, No. 12, p. 1072, "Guided-Wave Acoustooptic Bragg Modulators for Wide-Band Integrated Optic Communications and Signal Processing" by C. S. TSAI. With the disclosed configuration, though the drawback with regard to fluctuations of the optical amount of the optical wave in accordance with the deflection angle can be eliminated, the frequency of the surface acoustic wave must still be adjusted to be very high, and therefore the same problems as mentioned above occur.